Recently, positrons have received attention as a technology to detect micro-defects in a semiconductor device. Although a positron has a mass almost equal to that of an electron and a positive electric charge opposite to that of an electron, it does not exist stably in the natural world because the lifetime is as short as several-hundred picoseconds (one picosecond means 10−12 seconds), but it can be generated by a nuclear reaction. Defects in a material are negatively charged locally. Therefore, it is known that a low-energy positron bearing a positive electric charge is trapped at a defect site and is long-lived. After incidence into a sample, a positron collides with other electrons within several hundreds picoseconds and, by transforming masses of electrons and positrons to γ-ray energies, pairs annihilate each other producing γ-rays with an energy of 511 keV. 511 keV is a value obtained by conversion of masses of electron and positron into energy. Because an evaluation using positrons is a pulse-count measurement of high-energy γ-rays, high S/N and high sensitivity evaluations of micro-defects can be expected, which is different from one in which a probe itself becomes a background competing with a detected signal such as electron detection by electron beam irradiation. There is much research, and many presentations and patent applications for such evaluation of micro-defects using positrons.
When an evaluation method using positrons is applied to defect inspection of a semiconductor sample, the detectable size of defects is a problem. Technological developments therefore have focused on improvement of a convergent method to improve resolution. In the positron annihilation analysis apparatus described in JP-A 292380/2000, a method is disclosed, wherein a positron emitted from a positron source is transformed into a monochromatic and low-energy positron by passing through a thin film. Thereby, a low-energy positron with a uniform energy converges on a sample by electromagnetic lens. Now, by scanning the converged positron beam onto a sample using a scanning coil, the two-dimensional defect distribution on the sample surface can be evaluated. The sample surface is a surface with a discontinuity in atomic arrangement and an electronic state which is similar to that of a defect.
However, a positron has the characteristic of obtaining an energy equal to the work function on the sample surface, therefore it has a low probability of being trapped by an atom on the sample surface. Because of this characteristic, when a positron is injected into a so-called moderator thin film, a monochromatic positron is re-eliminated. Therefore, a positron converted to white light in the range of several hundred megaelectron volts becomes monochromatic below 0.5 eV, and the direction of emission is also oriented perpendicular to the film surface. Because of the low probability of trapping by an atom at the sample surface, it is expected that a sensitive evaluation is possible for micro-defects inside a target sample. There is an advantage which cannot be seen in microprobe techniques suitable for surface evaluation such as light, STM and AFM. However, the generating efficiency of a monochromatic positron is low because of a low re-emission probability of about 10−4.
On the other hand, in JP-A 74673/2001, a method to improve the S/N ratio (Signal-to-Noise ratio) of a detected signal, is disclosed, wherein γ-ray detectors are placed opposite each other and a coincidence measurement is carried out by an opposed detector-pair. According to this technique, a positron emitted from a positron source is focused by an electromagnetic lens and irradiated onto a sample. γ-rays generated by pair annihilation are detected by γ-ray detectors placed opposite each other. That is, using the characteristic that pair annihilation γ-rays are emitted simultaneously in opposite directions, taking the phenomenon that both detectors simultaneously detect as the true γ-ray signal, and taking the phenomenon that only one of them detects as background such as the electrical noise from cosmic rays and from the detector, it is possible to improve the SN. Thereby, information related to micro-crystalline defects with the size of several nanometers, that is a statistical distribution of defect size, can be evaluated.
FIG. 3 shows a schematic drawing of a positron defect evaluation instrument with the prior art. Positron beam emitted from positron source 9 becomes monochromatic by passing through a thin film of tungsten etc. with a clean surface, so-called moderator 14. If it is not monochromatic, the beam cannot converge due to the effect of chromatic aberration. A conventional positron source used for civilian applications often has a type in which a positron is obtained using beta-decay. In this case, the obtained energy distribution of a positron beam is continuous, and making it monochromatic is important. In order to converge a positron beam, electromagnetic lens 11 is used. A monochromatic positron beam is extracted from pick-up electrode 17 and converged onto sample 7 by electromagnetic lens 11. 15 is an electric power supply to provide an electrical potential difference at pick-up electrode 17, and 16 is an electric power supply to accelerate the positron by providing an electric potential difference between sample 7 and pick-up electrode 17. 12 is a means of scanning to scan the positron beam onto the sample surface, and 8 is a γ-ray detector. The γ-ray distribution, that is, the two-dimensional distribution of defect quantity, can be visualized by mapping the information of two-dimensional scanning position obtained from scanning means 12 and signal intensity detected by the γ-ray detector using data processing apparatus 13.
As described above, the points disclosed in the prior art are summarized as follows:
(1) Irradiating positrons into a sample, making them stay in defects, and extending their life at the defect positions.
(2) Generating γ-rays by pair annihilation of the positron losing energy at the position of the defect with the electron in the sample.
(3) Measuring the number of defects (density) from the γ-ray intensity.
(4) Scanning convergent positron beam onto the sample. Evaluating the two-dimensional defect distribution on the sample by synchronizing the information of scanned position with γ-ray intensity and drawing a two-dimensional intensity distribution.
(5) Making the positron beam monochromatic by passing it through a so-called moderator metallic thin film.
(6) Taking advantage of a nuclear reaction using an accelerator to strengthen the intensity of positrons.
(7) Using a beta-plus decaying sealed radioactive source such as 22Na, 64Cu, and so on, as a positron source.
(8) Applying a coincidence measurement using opposing detectors in order to improve the SN of γ-rays. That is, cosmic rays and exogenous background radioactive rays have almost no probability of being simultaneously detected by both detectors.
Therefore, SN can be improved by making an algorithm, wherein an event which is detected by both detectors is taken as a true event and an event not simultaneously detected is neglected.
In prior art, the two-dimensional distribution of size and number of micro-defects on the order of several nanometers could be measured statistically. The statistical distribution herein means average information inside a region of several hundred nanometers. This technique is applicable for evaluation of an object which exhibits a similar state across a wide area. However, for example, in an electrical device such as a semiconductor memory, wherein a transistor gate length and circuit structure of one bit which is the smallest unit of structure is less than 10 nm (in some cases, several nanometers), and when knowledge of the internal distribution is desired, realizing a spatial resolution less than 1 nanometer is required, and the problem occurs that the spatial resolution is inadequate by one to three orders of magnitude. Converging a positron beam onto a nanometer diameter size is thought to be in principle possible by making it monochromatic using a moderator and converging using an electron lens. However, monochromaticity makes the positron intensity about 1000 to 100000 times weaker than the positron intensity as emitted from the source, therefore the signal intensity of detected γ-rays becomes weaker, thereby creating a problem of inadequate measurement sensitivity. Measurements using an accelerator as a source have been attempted experimentally to maintain positron intensity, but there are problems such as cost and limitations on the number of measurements. It is therefore difficult to apply it to civilian products such as semiconductor devices. Thus, resolution and sensitivity are always mutually contradictory and a subject of the invention is to realize a measurement apparatus, a method of measurement, and an application apparatus using said measurement apparatus.